Monoethylene glycol is used as a raw material in the manufacture of polyester fibres, polyethylene terephthalate (PET) plastics and resins. It is also incorporated into automobile antifreeze liquids. Monoethylene glycol is typically prepared from ethylene oxide, which is in turn prepared from ethylene. Ethylene and oxygen are passed over a silver oxide catalyst, typically in the presence of an organic chloride moderator, producing a product stream comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water.
The product stream from the ethylene oxide reactor, which may also contain by-products such as organic chlorides, is then supplied to an ethylene oxide absorber. Typically, the ethylene oxide absorber has an initial quench section such as those described in U.S. Pat. No. 4,822,926 A and U.S. Pat. No. 5,336,791 A.
After the quench section, the product stream passes to the main section of the ethylene oxide absorber in which the ethylene oxide is contacted with and absorbed into a lean absorbent stream, which usually comprises water. The resultant aqueous ethylene oxide stream is referred to as ‘fat absorbent’ and is supplied to an ethylene oxide stripper, wherein ethylene oxide is removed as a concentrated aqueous ethylene oxide stream.
The ethylene oxide obtained from the ethylene oxide stripper may be undergo hydrolysis with water in order to provide monoethylene glycol and also higher glycols such as diethylene glycol and triethylene glycol. Preferably, the hydrolysis occurs in the presence of a hydrolysis catalyst in order to improve the selectivity to monoethylene glycol. Alternatively, the ethylene oxide may be reacted with carbon dioxide in the presence of a carboxylation catalyst to provide ethylene carbonate, and the ethylene carbonate may be reacted with water in the presence of a hydrolysis catalyst to provide ethylene glycol. Such processes are described in U.S. Pat. No. 6,080,897 A and U.S. Pat. No. 6,187,972 B1 and provide high selectivity to monoethylene glycol.
Homogeneous catalysts that are commonly used to promote hydrolysis of ethylene oxide or ethylene carbonate to ethylene glycol include basic alkali metal salts such as potassium carbonate, potassium hydroxide and potassium bicarbonate. The present inventors have found that during the operation of a process for preparing ethylene glycol, such hydrolysis catalysts can become deactivated during operation and that at least part of the deactivation occurs when chloroethanol, formed from the reaction of ethylene oxide with chlorides present in the EO reactor, reacts with the hydrolysis catalyst to form inorganic chlorides (such as KC1), which are inactive as hydrolysis catalysts, and ethylene glycol.
This deactivation means that it is necessary to provide additional fresh catalyst during operation. Further, a catalyst bleed stream must be employed in order to remove the deactivated catalyst, as well as other decomposition products from the process.
The present inventors have, therefore, sought to provide an improved process wherein the amount of chloroethanol present in the process is reduced and also wherein the decomposition of the hydrolysis catalyst is decreased or avoided.